EXTRACELLULAR GLYCOPROTEIN(S) ASSOCIATED WITH CELLULAR-DIFFERENTIATION IN MAGNAPORTHE-GRISEA

During the infection process of Magnaporthe grisea on rice plants, the conidial germ tube differentiates into a specific infection structure, an appressorium, for penetration into the host. Formation of appressoria was observed not only on plant leaves of various species including hosts and nonhosts, but also on synthetic solid substrata. We reported that the hardness of the solid surface contacted by conidial germ tubes was an important parameter for differentiation (Xiao ef al. 1994). Possible cellular factors involved in sensing solid surfaces and signal transduction during these early stages in pathogenesis were investigated. When germinated on a synthetic substratum, germ tubes and appressoria adhered firmly to the contact surface. Scanning electron microscopy showed abundant mucilaginous substances around germ tubes and appressoria. Conidial adhesion and appressorium formation were significantly inhibited by protease, alpha-mannosidase and alpha-glucosidase, but not by beta-glucosidase, alpha-galactosidase, lipase, and chitinase. The mucilage disappeared when germinated in the presence of protease, alpha-mannosidase, and alpha-glucosidase. Concanavalin A, a lectin binding to alpha-D-mannose and alpha-D-glucose, specifically suppressed appressorium formation at concentrations higher than 10 mu g/ml, but did not significantly affect conidial germination and adhesion. Mucilaginous materials were also observed around germ tubes and appressoria on various plant leaves. These data suggest that extracellular glycoprotein(s) bind germ tubes and appressoria to a contact surface, and at least a part of those glycoprotein(s) are further involved in sensing and transmission of information about the inductive parameters for cellular differentiation, The same mechanisms are probably involved in pathogenesis in vivo.